The short answer is yes. The slightly longer answer is, yes, but no two letter domains remain. Can you register something like d3.io? Yes. Can you register something like dx.io? No.

Last time I checked, there were some left without numbers. However they all had the letter Q in them, which people seem to hate.

Nope, not any more. Assuming your tool is correct (which, from checking other sources, it appears to be), all two letter io domains are taken, even those starting with q. There are still a decent number of letter + number domains [1], along with a ton of three letter domains.

[1] I posted a list of letter + number .io domains recently: https://news.ycombinator.com/item?id=5672237 - most are still available, but some were immediately taken due to the post and due to FramesPerSushi's tool (which I found rather useful).

You guys missed it! Back in January 2011, there were a bunch: http://rb.io/blog/two-letter-io-domains-available :-D